1. Field of the Invention
This invention relates to a surgically implanted spinal fixation device and method which permits improved correction of deformity and provides increased stability in the fixation of spinal fractures and other spinal deformities such as scoliosis and kyphosis.
2. Discussion of the Prior Art
The prior art includes a number of devices for use in correction of spinal deformities. Such devices are illustrated in U.S. Pat. Nos. 3,367,326; 3,426,364; 3,961,854 and 4,078,559, for example. One prior art device, disclosed in U.S. Pat. No. 3,426,364 uses a Y-shaped element which is connected to a body forming portion in order to replace one or more natural vertebrae.
Prior art devices have been used primarily for the correction of lateral deviation of the spinal column, known as scoliosis. The spinal curvature which results from scoliosis is generally defined on the basis of specific reference points. In particular, the extreme upper and lower vertebrae and the most displaced vertebra are of particular interest. The extreme upper and lower vertebrae are those which are the most inclined relative to the median longitudinal axis of the torso. The two planes within which the extreme upper and lower vertebrae can be found define the scoliotic angle. The most displaced, or apical, vertebra is defined as the vertebra which is the farthest from the median axis of the torso.
However, it is during surgery that the correction is completed and finalized. For this purpose, a solid rod with anchoring hooks is typically placed in the concavity of the curvature and a threaded rod with hooks is placed on the convexity of the curvature. These rods straighten the spine and maintain the correction until arthrodesis is attained by means of autogenous bone graft. The implants used most often to correct curvature during surgery are known as the Harrington.TM. distraction system and the Harrington.TM. compression system. Harrington is a trademark of Zimmer U.S.A., Inc., Warsaw, Ind.
Another use for spinal rods is correction of Kyphotic (hunchback) deformities produced by disease or spinal injury. In this use, the compression or distraction rods are used. Correction can be achieved by pushing the lamina of the apical vertebra anteriorly with the rods. However, particularly where the spine has a neutral or lordotic curve, there is a gap between the rods and spinal laminae. To the extent of this gap the spinal deformity cannot be reduced in such a way as to restore or maintain anatomic lumbar lordosis, and full indirect spinal cord decompression often cannot be achieved. Moreover, since the anterior longitudinal ligament and other soft tissues attached to the vertebrae anteriorly are not placed under maximal stress in such a case, maximum stability of fixation is not accomplished either.
Improved three-point fixation can be achieved by bending of such rods. However, clinical experience has shown that despite wire and other methods of splinting the bent rods, they can rotate out of position.
One prior art solution to the problem of bent rods rotating out of position has been to use square hole hooks and rods. The use of square hole hooks and rods has four disadvantages, however:
1. A hospital is required to double its stock of rod equipment;
2. The bending technique weakens the rod;
3. The technique requires considerable time and skill to obtain the ideal amount and location of the bend during surgery;
4. Many rods can be wasted in achieving the ideal bend during the operation.
Thus, the prior art devices do not provide for spine stabilizing devices which provide enhanced pressure distribution against the spinal vertebrae, high stability against rotation, or easily convertible dimensioning to accommodate differently sized patients and patterns of spinal injury or deformity.